1. Field of the Invention
The invention relates to the field of self-sensing piezoresistive nanocantilevers used as biosensors and methods of optimized biasing self-sensing piezoresistive nanocantilevers.
2. Description of the Prior Art
Silicon microscale and, more recently, nanoscale cantilevers enable important applications such as atomic force microscopy (AFM) and biological force spectroscopy. Most efforts in this area employ cantilever probes with external displacement transduction via off-chip sensing systems. These systems are typically optically-based, involving simple optical beam deflection or more sensitive interferometry. Self-sensing cantilevers, which possess integrated displacement transducers, offer important advantages that are not attainable with external optical methods. Perhaps most prominent are: scalability to extremely small cantilever dimensions (far below an optical wavelength) and, thereby, to very high frequencies; measurement without optical perturbation of susceptible samples; suitability for large-array technologies and portable sensing; and ease of applicability to multiple-cantilever sensors that permit correlated or stochastic detection. Furthermore, use of on-chip electronic readout is especially advantageous for detection in liquid environments of low or arbitrarily varying optical transparency, as well as for operation at cryogenic temperatures where maintenance of precise optical component alignment becomes problematic.
Emerging forefront applications such as magnetic resonance force microscopy (MRFM) of single spins, and BioNEMS (biofunctionalized NEMS) for single-molecule biosensing; require compliant mechanical nanosensors with force sensitivity at the thermodynamic limit. A milestone along the path towards ultralow noise, self-sensing devices is the work of Harley and Kenny who demonstrated piezoresistive microcantilevers achieving a force noise spectral density of 8.6 fN//√Hz in air at a frequency of about 1 kHz with extremely compliant, 30 pN/m devices. More recently, Bargatin et al. report measurements of piezoresistive nanocantilevers operating at very high frequencies, up to about 71 MHz, attaining a force sensitivity of 350 aN/√Hz in vacuum at room temperature.
Nanoelectromechanical systems (NEMS) have advantages of matching the biological system both in size and in speed. However, it is not a simple task to observe the biological signals from these sensors. Due to the ultra-small size of the nanoscale sensors, the signal generated by individual nanosensor is generally exceedingly small. This tiny signal is also vulnerable to the large parasitic signal from the electrochemical environment.